The childhood-onset spinal muscular atrophies (SMA) are a recessively inherited group of disorders characterized by degeneration of the spinal cord anterior horn cells with concomitant muscle weakness and atrophy. The childhood SMAs are often divided into three clinical classes based upon severity of illness and age of onset with SMA type I (Werdnig- Hoffmann disease) as the most severe form (death usually by 2 years), followed by Intermediate SMA (type II), and with the mildest and most heterogeneous manifestation of illness classified as SMA type III (Kugelberg-Welander disease). The childhood SMAs constitute the most frequent heritable form of infant death with an incidence of 1:10,000. The disease locus for all three SMAs has been genetically mapped to a 1-2 cM region believed to span approximately 400-500 kb of chromosome 5q13. Several lines of evidence support the notion that the disease gene is located in a subset of this region characterized by multiple, low-copy number repeat sequences typified by multi-locus microsatellite markers and multi-copy genes and/or pseudogenes. YAC contigs that span the repeat- rich region are prone to delete some, or all copies of these multi-locus markers. Interestingly, the same multi-locus markers detect significant linkage disequilibrium to the SMA disease locus and also appear to be preferentially deleted in SMA patient DNA (particularly SMA type I). Positional cloning of the SMA gene has been hindered by the DNA sequence complexity and concomitant instability of clonal DNA. In this proposal we outline a cloning strategy that deals with the unique problems of this region and seeks to take advantage of the genomic instability to identify and clone the SMA gene. The second half of the proposal describes the characterization of the SMA gene, evaluation of mRNA expression, and preliminary study of the protein product. We explore the relationship between disease-mutations and clinical manifestation of the illness as a strategy to relate gene function to disease pathology. Finally, we outline a gene "knock-out" strategy to study gene expression, disease pathophysiology, and possible treatment of the disorder.